ES2383747A1 - Method for forming cellular pet from solid preforms - Google Patents

Abstract

The method comprises the steps of: (a) conditioning a solid preform (1) made from PET (polyethyleneterephthalate), previously obtained by extrusion, moulding or heat-forming, adjusting the moisture concentration thereof to the concentration value thereof at equilibrium (b) dissolving, in said solid preform (1), an inert gas, at pressure and temperature, for a sufficient period to achieve saturation, and decompression at ambient temperature, thereby resulting in the gas-saturated solid preform (1) and (c) placing the gas-saturated solid preform (1) in a non-metallic forming mould and applying microwave radiation to the whole in order to produce softening, expansion and simultaneous forming of the solid preform (1), until a cellular product (6) is obtained.

Description

PROCEDURE TO CONFORM CELL PET FROM PREFORMS

SOLIDS

Field of the art The present invention concerns a method of forming cellular PET (polyethylene terephthalate) from solid preforms.

Background of the invention

Methods based on dissolving an inert gas, commonly CO2 or N2, are known in a plastic component for a time at a certain temperature and pressure to then apply external heating to the product and thus generate its expansion by volatilization of the dissolved gas. The formed foam is cooled in a subsequent stage. This process is commonly known in the state of the art as solid state foaming or two-stage batch foaming.

US-A-4473665 [1] defines the main points of a method for producing two-stage batch polymeric foams: a first stage comprises saturating a solid polymeric preform under high pressure with an inert gas used as a physical foaming agent; and a second stage comprises heating the polymer preform saturated with the gas to a temperature higher than its glass transition temperature and subsequently stabilizing the foam by cooling.

US-A-4456571 [2] contemplates a process and an apparatus for obtaining foamed polymeric filaments with micrometric cell sizes. The main difference with respect to the previous patent is that it comprises a chemical foaming process, that is, by thermal decomposition of chemical additives designated as chemical foaming agents, by melt extrusion, and cooling of said foamed filament obtained at the exit of the nozzle of the extruder by contact with cooled rollers. This procedure has as its main drawback the need for a high pressure drop at the exit of the extrusion nozzle to generate a cellular structure of high cellular density, with a consequent reduction in production. Thus, for example, in conventional extruders, where melt pressures of up to 250 bar can be reached, extrusion nozzles of 0.46 mm in diameter and 12.75 mm in length are required, resulting in extremely low productivity [3 ].

Patent ES-A-2236720 T3 [4] discloses an apparatus and a method for obtaining polymeric foamed components by semicontinuous methods. In particular, it refers to foamed components with a closed cell cell structure with typical cell sizes <10 IJm (cell density> 108 cells / cm 3). The patent contemplates the process and the apparatus for producing continuous sheets or strips of foamed cell polymers using a two-stage batch foaming process, with an initial stage of dissolving an inert gas, commonly CO2 or N2, at high pressure in a solid polymer, and subsequent foaming by heating at atmospheric pressure at a temperature higher than the glass transition temperature of the gas saturated polymer sample. Once the desired expansion is achieved, which is achieved based on the amount of gas dissolved in the first stage and the foaming time in the second, the foam produced is cooled to stabilize its cellular structure.

The main novelty of this patent ES-A-2236720 T3 [4] against, for example, US-A-4761256 [5], which discloses a method for continuously obtaining foamed polymeric tapes with smooth skins solid (integral foams), where said foams are achieved by heating plastic belts previously impregnated with gas by an extrusion process in the molten state or in an autoclave at high pressure in the solid state, is that it does not require high-pressure chambers, very expensive due to to the required tightness, and allows the production of foamed sheets of superior thickness.

To do this, the aforementioned patent ES-A-2236720 T3 [4] describes a process that comprises, in a first stage, saturating a rolled sheet of solid polymer next to sheets of a gas permeable material arranged intercalated, at high pressure with a gas at room temperature (typical values for CO2 between 5 and 70 bar, although values of up to 100 bar can be reached, for a time between 3 and 100 hours) and, after decompressing it, unwind the material and separating the gas saturated polymer sheet from the permeable sheet, and finally applying heat to said gas saturated polymer sheet for a time deemed necessary to achieve a desired expansion (foaming temperature between 80 and 200 OC). One of the differences provided is that tension means are applied on the polymeric film saturated with gas during this heating stage to avoid the formation of wrinkles during foaming.

In the aforementioned patent ES-A-2236720 T3 [4] the homogeneity of the cellular structure is achieved by means of a uniform concentration of dissolved gas throughout the entire sample as a result of an optimization of the gas dissolution conditions, this it is, dissolution time, temperature and pressure, in order to guarantee the uniform dissolution of the gas throughout the thickness of the solid precursor. However, it still requires a heating stage to generate the expansion of the material, which considerably limits the production of foamed sheets of high thickness. In particular, the application of heat, either by contact of heating plates on the upper and lower faces of the gas-saturated plastic sheet, or by a radiant heat source (eg infrared), or by immersion in a fluid bath heated, it generates a temperature gradient between said upper and lower faces and the core of the sheet due to the thermal insulating nature of plastics, which tends to cause foams with slightly homogeneous cellular structures, with larger cells in the upper and lower areas (more temperature) and even, for larger thicknesses, foams with almost solid central areas (see Fig. 1, State of the art). Also, the saturation times of the gas in the solid sheet are very high. For example, and for a 0.51 mm thick PET (polyethylene terephthalate) sheet, the time for saturation of said sheet with CO2 is between 15 and 30 hours.

As can be seen, there is a need in the art to simplify and optimize procedures that allow to obtain foamed polymer sheets of high thickness with a homogeneous cellular structure therethrough and of small cell size.

Since plastic materials are good thermal insulators, these processes have the disadvantage that the external heating applied is not transmitted uniformly through the thickness of the product, but in a variable way, creating temperature gradients of many degrees from the surface to the core . Consequently, the expansion that occurs is not uniform, nor is the cellular structure of the foam that is formed. That is why the aforementioned procedures are neither satisfactory nor recommended for obtaining high thickness foams. Thus, when it is intended to obtain a foam from a product of high thickness (typical values> 1-6 mm), the expansion is not uniform due to the local temperature gradient that is created through the thickness in the external heating stage. Whether an external heating is applied by contact or by convection with a fluid, as if it is carried out by heat radiation, the temperature reached on the surface of the material (skin), so that it is adequate in the central line (core), is much higher than recommended to create a foam. This results in a deficient and non-uniform expansion of the material, which manifests itself in the form of high density values (low degree of expansion and porosity), large cells formed by coalescence, and skins with the same density as the solid, formed by cell collapse In addition, the material of the skins, in this case, undergoes thermal or oxidative degradation due to excessive heating. This implies the need to incorporate protective additives in the formulation of the material (stabilizers and antioxidants), which make the product more expensive. At the other end, if external heating is applied more moderately so as not to overheat the skin of the product, what is achieved is not to transmit enough heat to the core so that its expansion occurs, obtaining foams with a density gradient and cellular characteristics ranging from skins with minimal density to the core with a density as high as that of solid material (Fig. 1, State of the art).

A possible solution to this problem is to achieve foaming of the gas-saturated polymer using other procedures. In particular, we establish that microwave radiation is the alternative solution to external heating by convection with a fluid or by contact with heating plates in order to generate a uniform cellular structure throughout its entire thickness (see Fig. 2 ).

Methods that consider microwave radiation in polymer foaming processes are known in the state of the art. In particular, US-A-5118722 [6] contemplates the preparation of flexible polyurethane foams for acoustic insulation applications in the automotive sector using microwave radiation during the foaming stage. The main difference with respect to the process of the present invention regarding the use of microwave radiation is that in the said US-A-5118722 [6], the foaming is done from the liquid mixture of the two monomers (isocyanate and polyol) which by polymerization reaction produce a thermostable polymer (polyurethane). Said polymerization reaction generates gas as a secondary product within the reaction, which acts spontaneously as a foaming agent for the polyurethane that is formed, thus obtaining a thermosetting polymer foam. Microwave radiation serves here to stimulate the vibration of the molecules in two reactive components and thus facilitate the chemical reaction between them. Once the desired expansion is achieved, the stabilization of the cellular structure is achieved by controlled cooling. In the process of the present invention, a mixture of the liquid monomers is not used to achieve a foam of thermosetting material, but of a thermoplastic solid precursor previously saturated with an inert gas and, therefore, the foaming by radiation application of Microwave is made at all times with the solid state material, with inherent advantages. These include the prevention of problems related to non-uniform foaming by improper mixing of the components of the formulation, which tends to worsen the higher the final thickness of the foam.

Two great methods or methods for forming products with cellular thermoplastic polymers, in particular PET, are known in the state of the art. On the one hand, injection molding, in which a molten mass of the polymer at a high temperature and with a determined concentration of dissolved gas is introduced at high pressure into a mold. The depressurization of the melt inside the mold generates the release of the gas dissolved in the polymer with the consequent expansion or foaming, while the cooling of the mold is responsible for the stabilization of the foam with the desired shape. On the other hand the thermoforming, method in which the foam in the form of a sheet or roll is subjected to direct contact heating, radiant heat or convection with fluid in a first stage to produce its softening. At a later stage, the foam is formed by the action of forces on an open mold.

There is therefore a need in the art to simplify and optimize procedures that allow obtaining high thickness foamed PET products and a homogeneous cellular structure, together with advantageous procedures for the production of cellular polymer components with complex geometric shapes by simultaneously conforming to the expansion of said material.

Exhibition of the invention

The present invention helps to alleviate the disadvantages and disadvantages of the state of the art by providing a method for forming cellular PET (polyethylene terephthalate) from solid preforms comprising the steps of:

(to)

conditioning a solid preform previously obtained by extrusion, molding or thermoforming, adjusting its equilibrium moisture concentration «1% depending on temperature and relative humidity).

(b)

dissolve in the preform an inert gas at pressure and temperature for sufficient time until saturation, and decompression at room temperature, whereby the saturated solid gas preform is obtained; Y

(C)

place the saturated gas preform between the plates of a non-metallic mold and apply a microwave radiation on the assembly to produce the softening, expansion and simultaneous forming of the preform.

The preform can be a sheet, plate, bar, tube, profile or previously formed or machined shape, which preferably has a fundamentally amorphous microstructure and a thickness between 1 and 9 mm.

In the process, the PET preform is preferably subjected to conditioning for 72 hours at a temperature between 20 ° and 60 ° C and a relative humidity between 50% and 95%, whereby the preform reaches its moisture concentration in the Balance.

The inert gas used can be, for example, argon, nitrogen, carbon dioxide, or any of its mixtures.

In an exemplary embodiment, the solid PET preform is subjected to carbon dioxide at a pressure between 100 and 300 bar for a period between 20 and 120 min and at a temperature between 40 and 120 ° C.

The process contemplates, in one embodiment, the expansion of the solid preform saturated with gas by applying microwave radiation, and more specifically a microwave radiation of typical frequency between 1 GHz and 30 GHz for a time between 1 and 30 My no

Simultaneously with the expansion of the preform, the method optionally contemplates forming the cellular product between plates of a mold, by the action of the expansive force of the material of the preform.

Brief description of the drawings

The foregoing and other features and advantages will be more fully understood from the following detailed description of some embodiments with reference to the attached drawings, in which:

Fig. 1 is a diagram illustrating a prior art procedure using external heating sources;

Fig. 2 is a diagram illustrating the method of forming cellular PET (polyethylene terephthalate) from solid preforms according to an embodiment of the present invention; Y

Fig. 3 is a diagram illustrating the result of using microwave radiation on a solid preform in the process of the present invention.

Detailed description of some embodiments

Referring first to Fig. 1, there is illustrated a process known in the state of the art, which comprises heating a sheet-shaped body 10, made of a polymer such as PET (polyethylene terephthalate) and saturated with an inert gas, by means of external heating sources to achieve a foamed material by polymer expansion. In the example of the upper left part of Fig. 1 the body 10 is heated by a pair of heating plates 12, 14 applied to opposite surfaces 10a, 10b thereof. In the example of the lower left part of Fig. 1 the body 10 is heated by induction by means of a heating bath 16 in a container 18.

In both cases, as shown in the right part of Fig. 1, the same result is obtained, this is a foamed body with a non-uniform cell size, where due to the temperature gradient between said opposite surfaces 10a, 10b and a core 10c of the sheet-shaped body 10 produced by thermal insulating nature of the plastics, the cells adjacent to the opposite surfaces 10a, 10b are comparatively larger because they have received more temperature and the cells closest to the core 10c are comparatively smaller or Even if the body 10 is very thick, the cells are absent and the central area is practically solid.

With reference now to Figs. 2 and 3 a process for producing PET shaped parts (polyethylene terephthalate) with a cellular structure (foam) from solid preforms according to an embodiment of the present invention is described.

As shown in Fig. 2, the procedure consists of several stages: (a) conditioning of a solid PET preform 1; (b) dissolution of a gas until saturation; and (c) formed by microwave radiation application.

In step (a) of conditioning, the solid preform 1 or solid precursor, which has been previously obtained by extrusion, molding or thermoforming of PET, has a totally amorphous microstructure because it has been rapidly cooled from the molten state, that is, preventing polymer crystallization. The solid preform 1 has a thickness between 1 and 9 mm, which is selected according to the thickness sought for the foamed component to be obtained. The conditioning step (a) comprises maintaining the solid preform 1 for 72 hours in a chamber of an oven 2 at a temperature between 20 ° and 60 ° C in an atmosphere of relative humidity between 50% and 95%, for ensure that the moisture content of the preform reaches an equilibrium concentration.

The step (b) of dissolving the inert gas (preferably CO2) in the solid preform 1 is carried out inside an autoclave 3, that is, a tight tank under high pressure, and at a temperature between 40 and 120 ° C for a time. between 20 and 120 minutes. The conditions are chosen based on the thickness of the solid preform 1 and the nature of the polymer, as well as the concentration of gas to be dissolved inside. As regards gas saturation pressures, pressures between 100 and 300 bar are considered adequate, using special compressor pumps for this purpose to increase the gas supply pressure of the cylinders. The gas pressure inside the autoclave 3 will be selected based on the initial thickness of the solid preform 1 or solid precursor, as well as the final density and cellular structure of the foam to be obtained. Generally, the higher the thickness of the solid preform 1, the more gas dissolution time will be necessary. The temperature, pressure and dissolution time of gas in the polymer are regulated on purpose to obtain typical values of gas concentration in the precursor between 20-70 mg / g of PET. After the saturation time with the inert gas, controlled decompression and extraction of the solid preform 1 of the autoclave 3.

The third stage (c) of forming by microwave radiation application comprises applying on the solid preform 1 saturated with gas obtained in the previous stage, which has previously been arranged in a non-metallic mold, a microwave radiation with a typical power between 400W and 800W for a sufficient time (typically 1-30 min) for its softening and expansion, thus forming a cellular product 6 (foamed body), which when expanded will fill all the cavities of the mold and therefore the cellular product 6 resulting will have the configuration provided by the mold geometry (Fig. 3). To facilitate the release, it is convenient that the mold is configured to provide smooth shapes, such as, for example, a flat plate shape, a wavy plate shape, or the like.

In the embodiment shown in Fig. 2, the mold comprises two plates 4, 5 located on opposite sides of the solid preform 1 and configured to provide a cellular product 6 in the form of a corrugated plate. The mentioned plates 4, 5 are non-metallic so as not to interfere with microwave radiation.

Between the stage (b) of dissolution of the inert gas and the stage (c) of forming it is convenient to allow sufficient time to allow a stabilization of the concentration of the inert gas throughout the entire thickness of the solid preform 1, with that the solid preform 1 is prevented from experiencing a sharp expansion when irradiated with microwaves. It is also important to measure the microwave radiation power and exposure time well to avoid excessive radiation.

Fig. 3 shows the cellular product 6 resulting from the application of the process of the present invention, in which the foamed polymer has a substantially uniform cell size throughout the entire thickness of the body.

Thus, unlike in the procedures of physical foaming of plastics (for example, microcellular foaming in solid state), in which the expansion of the saturated gas material occurs by direct application of heat, either in a water bath, glycerin or silicone, or by thermal radiation, in the process of the present invention the use of an external heat source is not contemplated but rather the degree of softening of the necessary material and its expansion are achieved by application of low-level microwave radiation. medium frequency (1-30 GHz).

In accordance with the process of the present invention, obtaining the shaped cell product 6 is achieved with a particular combination of several factors: (i) a minimum concentration of dissolved water molecules in the saturated gas polymer of 0.1% in weigh; (ii) microwave application for a suitable time and power and (iii) a mold geometry used.

Modifications, variations and combinations will occur to one skilled in the art from the embodiments shown and described without departing from the scope of the present invention as defined in the appended claims.

Particular examples Example 1: In a particular application, a solid circular PET preform of density 1.31 g / cm3 and dimensions: diameter 64 mm and thickness 2 mm, obtained from a flat sheet obtained previously by an extrusion process is arranged. The preform is conditioned in an oven under a controlled atmosphere of relative humidity 80% and temperature 50 oC for 72 hours, after which it is introduced into a reactor with temperature and pressure control where it is subjected to a CO2 pressure of 100 bar and a temperature of 90 oC for 120 minutes. After this period, the reactor temperature is reduced to 25 ° C by means of water recirculation and the necessary decompression is carried out in order to open the device and extract the preform, which is then saturated with CO2, as indicated by the weight gain with regarding its initial reference value.

The saturated preform is placed between the two halves of a mold formed by two plates that make up a cylindrical space 65 mm in diameter by 13.2 mm thick, and the assembly is introduced inside a microwave oven where the transformation takes place , consisting of the simultaneous expansion and shaping of the saturated preform in a flat product with cellular structure, due to the interaction of a microwave radiation of frequency 2.45 GHz and power 400 W for a period of 10 minutes with the particular chemical composition of the preform. The piece thus formed has dimensions of 64 mm in diameter by 13.2 mm thick, and has a closed cell structure characterized by an average cell size between 0.1 and 0.2 mm, and a degree of orientation of the cells in the direction of the growth (vertical or in thickness) of 1.2, defined as the ratio between the average cell size in the vertical and horizontal direction. Its density is 0.20 g / cm3, which represents that it has undergone a degree of expansion of 6.2, defined as the ratio between the density of the solid preform and the density of the shaped cellular product.

Example 2: In another particular application, using the same conditions as in example 1 but applying in this case a microwave radiation power of 600 W (frequency of 2.45 GHz), the saturated preform of CO 2 is caused a further expansion fast that produces a degree of cellular coalescence and densification of the skins in contact with the internal walls of the forming mold, resulting in a shaped piece of structural cellular PET with integral skins 0.6 mm thick, density 0.40 g / cm3 and structure cell with average cell size of 0.3-0.7 mm and a degree of orientation in the vertical direction.

Example 3: In another application, once the PET preform is conditioned under the same conditions as indicated in example 1, CO 2 is dissolved therein under a pressure of 250 bar at a temperature of 90 ° C for 60 minutes. Unlike example 1, the simultaneous expansion and shaping stage by microwave radiation application is performed with a power of 800 W for 2 minutes, resulting in a hollow and fully closed PET body, rather than a foam , due to the occurrence of a rapid expansion that causes complete coalescence or breakage of all initially formed cells. The product thus obtained has rigid skins of typical thickness 0.7 mm and a density of 0.10 g / cm 3.

References

[1] Martini-Vvedensky, J.E., Suh, N.P., Waldman, F.A. Microcellular closed cell foams and their method of manufacture, U.S. patent 4,473,665 (1984).

[2]

Johnson, O.E. Process and apparatus for forming a composite foamed polymeric

sheet structure having comparatively high density skin layers and a comparatively

low density core layer, U.S. patent 4,456,571 (1984).

[3]

Throne, J.L. Thermoplastic Foam Extrusion, Hanser Gardner Publications,

5

Cincinnati (2004).

[4]

Kumar, V., Schirmer, H., HolI, M. Semi-continuous production of polymeric foams

in solid state, patent ES 2236720T3 (2005).

[5]

Hardenbrook, S.B., Harasta, L.P., Faulkenberry, S.T., Bomba, R.o. Method for

producing microcellular foamed plastic material with smooth integral skin, U.S.

10

patent 4,761,256 (1988).

[6]

Wollmann, K., Ach, A., Frank, W. Method for producing elastic foams having a base

of polyurethane by microwave foaming, U.S. patent 5,118,722 (1992).

[7]

Pip, W. Method for foaming synthetic resin bodies with microwave or high frequency

energy, U.S. patent 4,740,530 (1988).

fifteen

Claims (9)

1.-Procedure for forming cellular PET from solid preforms, comprising the following steps:

(to)

conditioning a solid preform (1) made of PET (polyethylene terephthalate) previously obtained by extrusion, molding or thermoforming, adjusting its moisture concentration to its equilibrium concentration value;

(b)

dissolve in said solid preform (1) an inert gas at pressure and temperature for a sufficient time until saturation, and decompression at room temperature, whereby the solid preform (1) saturated with gas is obtained; Y

(C)

place the solid preform (1) saturated with gas in a non-metallic mold and apply a microwave radiation on the assembly to produce the softening, expansion and simultaneous shaping of the solid preform (1) until obtaining a cellular product (6) .

2. Method according to claim 1, characterized in that the solid preform (1) of PET (polyethylene terephthalate) has a fundamentally amorphous microstructure and a thickness between 1 and 9 mm. 3. Method according to claim 1 or 2, characterized in that the solid preform (1) of PET (polyethylene terephthalate) is a sheet, plate, bar, tube, profile previously formed or machined. 4. Method according to claim 1, characterized in that the solid preform (1) of PET (polyethylene terephthalate) is subjected to conditioning for 72 hours at a temperature between 20 ° and 60 ° C and a relative humidity between 50% and 95% by which reaches its concentration of moisture in the balance. 5. Method according to claim 1, characterized in that the inert gas is selected from argon, nitrogen, carbon dioxide, or mixtures thereof. 6. Method according to claim 1, characterized in that the solid preform (1) of PET (polyethylene terephthalate) is subjected to carbon dioxide gas at a pressure between 100 and 300 bar. 7. Method according to claim 1, characterized in that the solid preform (1) of PET (polyethylene terephthalate) is subjected to carbon dioxide gas for a period between 20 and 120 min. 8. Method according to claim 1, characterized in that the solid preform (1) of PET (polyethylene terephthalate) is subjected to carbon dioxide gas at a temperature between 40 and 120 oC. 9. Method according to claim 1, characterized in that the expansion of the solid preform (1) of PET (polyethylene terephthalate) is produced by applying microwave radiation of typical frequency between 1 GHz and 30 GHz for a time between 1 and 30 min. Method according to claim 1 or 9, characterized in that simultaneously with the expansion of the solid preform (1) of PET (polyethylene terephthalate) the cellular product (6) is formed between plates (4, 5) of a mold, by action of the expansive force.  12 lOa License plate. <~ l.f ~ {tons B ~ ilod. <'; ¡Nhuni. "T. (to) (} iPHnsl.;. n ~~ '.j c (> n {.; :: ntt:} (i · ~ ndt tbO ~ nl¡) 1: IH: forl; l ~ 2 E! IpaM-a MI ulfonFlepar Nd! EAled. ", .., .... t> =. lOa lOe lob Figure 1 (b) (e) l.! Pr ~ $ lI)! L and t'-nnro; d.o :: :: - ~ I ~ n'l {j '~ n do :: (02 ~ (~ f, - '{r: ~ iJ': -¡ ::: tJlJ ~ iiJ (J '~ 8; 0 Y (one): 3 Figure 2 N1ic! Eo <dollar (u '...... h) Figure 3